Heard exact changes from an authoritative source (2,8)

I believe the answer is:

ex cathedra

'from an authoritative source' is the definition.

'heard exact changes' is the wordplay.
'changes' indicates an anagram.
'heard'+'exact'='heardexact'
'heardexact' with letters rearranged gives 'EX CATHEDRA'.

(Other definitions for ex cathedra that I've seen before include "on good authority" , "with papal authority" , "officially" , "With the authority of office (Latin)" .)

I've seen this clue in The Times.

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